Why does squinting eyes help make an image sharper? Why aren’t we always squinting, and thus by definition, not seeing as clearly as possible?
Ugh sorry for butchering the English there…it’s late 
Reduces the ‘window’ the eye can see out of, making focusing easier. Try poking a small hole in a piece of paper and putting it up to your eye. You’ll have an amazing superpower!
–Tim
Yep, smaller aperture, higher resolution. Added to this the brain is for some reason better at deciphering and recoding patterns into something recognisable if the image isn’t clear. No one’s quite sure why (last I heard) but this is the reason those pixelated images they use to stop offenders being identified on shows like ‘COPS’ don’t work real well if you squint. You can still identify the person, particularly if you know them.
Why don’t we squint all the time? Firstly it reduces your field of vision, so anything approaching from the sides is invisible. Secondly the smaller aperture also means less light, and therefore a weaker image for the brain to process. Try squinting while looking into a dark area of the room you’re in, it should all but vanish. Squinting only works well if you’re looking at something that’s brightly lit. If you squinted all the time most shadowed areas would appear pitch black.